OSPF DR Election Rules and Priority Configuration

H12-811 By islever Editorial Last updated

OSPF Principles and Configuration

Short answer

D A router with an interface priority of 0 is ineligible to become the OSPF Designated Router.

Question

Which of the following statements is true about the OSPF DR?
  1. The DR must be the device with the highest priority on the network.
  2. The DR is elected in preemption mode.
  3. A router with a larger router ID is elected as the DR.
  4. If the priority of an interface is 0, the device where the interface resides cannot become the DR. Correct Answer

Key Takeaways

The main trap is assuming Router ID alone determines the DR or that preemption occurs; in reality, priority is king, and elections are non-preemptive.

Understand OSPF DR election mechanics where interface priority takes precedence over Router ID, and priority 0 excludes a router from DR eligibility.

Common mistake: Selecting Option C because users often forget that Router ID is only a tie-breaker for equal priorities, not the primary determinant.

Expert Analysis

The OSPF DR (Designated Router) election process on broadcast or NBMA networks follows a strict hierarchy of criteria to ensure stability and predictability. The interface priority (ranging from 0 to 255) is the primary factor. A router with the highest priority becomes the DR. If priorities are equal, the Router ID serves as the tie-breaker, where the highest Router ID wins.

Option D is correct because setting the interface priority to 0 explicitly disqualifies the router from participating in the DR/BDR election. It remains an OSPF neighbor but cannot become the DR or BDR. This is a common configuration technique to force specific routers into backup roles or exclude them entirely.

Option A is incorrect because while the highest priority usually wins, it is not an absolute requirement if another router has an equally high priority but a higher Router ID. Thus, 'must' makes this statement false.

Option B is incorrect because OSPF DR election is non-preemptive. Once a DR is elected, even if a new router joins with a higher priority, it will not take over the DR role unless the current DR fails or the OSPF process restarts. This prevents unnecessary flapping of roles.

Option C is incorrect because the Router ID is only used as a secondary criterion when priorities are tied. A router with a high Router ID but low priority will not be elected DR over a router with a lower Router ID but higher priority.

Official Reference

https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/13688-21.html

This link opens the official vendor documentation in a new tab.

Exam Strategy

When analyzing OSPF election questions, always check the interface priority first. Remember that OSPF is non-preemptive, meaning the initial DR stays unless it fails. Use priority 0 to prevent a router from becoming DR without shutting down its OSPF process.

Frequently Asked Questions

Does OSPF DR election happen automatically?

Yes, on broadcast and NBMA networks, OSPF automatically elects a DR and BDR during adjacency formation based on priority and Router ID.

Can a router with priority 0 become the BDR?

No. Setting the interface priority to 0 disqualifies the router from both DR and BDR roles. It can only participate as a DROTHER.

More H12-811 FAQ →

Discussion 7 contributions

Compiled from public community threads and reader submissions, lightly edited for clarity. Handles and vote tallies are shown as recorded, and comments reflect the views of individual contributors.

From public community threads 7

  1. hafizhou 👍 4
    D is correct
  2. Fmoura17 👍 1 Answered D
    D. If the priority of an interface is 0, the device where the interface resides cannot become the DR. A ❌ – The highest priority wins, but if priorities are equal, the highest Router ID breaks the tie. Highest priority is not strictly required. B ❌ – OSPF DR election is non-preemptive. Once a DR is elected, it keeps the role even if a higher-priority router joins later. C ❌ – Router ID is only a tiebreaker when priorities are equal, not the primary criterion. D ✅ – A priority of 0 explicitly disqualifies a router from becoming DR or BDR. This is by design, allowing admins to manually exclude devices from the election.
  3. konyuz 👍 1 Answered D
    Priority: The interface priority (0–255) is the first criterion. Highest priority wins. Router ID: If multiple routers have the same highest priority, the one with the highest router ID becomes DR. Preemption: OSPF DR election is non-preemptive. Once a DR is elected, a new router with a higher priority joining later will not replace it until the current DR fails. Priority 0: If an interface has priority = 0, it is ineligible to become DR/BDR.
  4. foetus 👍 1
    Correct. While A is partially correct, if there is a tie on the router priority. Then the highest router ID is selected as the DR. So potentially both are needed to decide the DR. While if the priority if an interface is 0, the device where the interface is connected to (resides) cannot become the DR!
  5. Chleong 👍 1 Answered A
    Highest priority win
Show 2 more comments
  1. akamg 👍 1 Answered A
    Option A is correct
  2. js293 👍 1 Answered D
    D is more complete

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